The following publications are representative of the most relevant prior art known to the Applicant at the time of filing of the application.
______________________________________ United States Patents ______________________________________ 3,560,441 February 2, 1971 M. Schwarcz et al 3,953,650 April 27, 1976 E. S. Sauer et al 4,089,912 May 16, 1978 R. P. Levek et al 4,182,799 January 8, 1980 J. Rodish ______________________________________
As the use of plastics has grown in recent years in such areas as construction, automobiles and aircraft, so too has the concern over the flammability of these materials. To respond to that concern, the industry has discovered and introduced into the plastics or polymers various fire retardant materials either singularly or as a flame retardant systems made of several materials. While the currently used flame retarding materials have brought about a significant decrease in the flammability of many of the more widely used polymers, there is still substantial room for improvement. It is this specific problem which the present invention addresses.
The Schwarcz et al reference discloses a flame and smoke retarding system for polymers such as polyethylene, polyproplyene, polyesters, polyvinyl chloride and the like. The system is made up of antimony trioxide and one or more compounds from the group sodium borate, lithium borate, zinc borate, ammonium borate, potassium orthophosphate, sodium phosphate, sodium fluoro phosphate, lead oxide, sodium aluminate, sodium aluminum fluoride, boric anhydride, phosphorous pentoxide, sodium pyrophosphate, barium oxide, sodium carbonate, cobaltous carbonate, calcium oxide, magnesium oxide and sodium antimonate. The inorganic compounds in powder form, are mixed and then heated to fusion. The resultant glass is pulverized and incorporated in the polymer in the conventional manner. The reference compares its flame retardant system with antimony trioxide alone, compounded with polyvinyl chloride. The flame retardancy effect of all of the additions to the polyvinyl chloride was about the same, measured by how far the flame spread and how much smoke was generated. However, the flame retardant of the reference did reduce the degree of yellowness which occurred in polyvinyl chloride samples upon heating.
Clear polyvinyl chloride is made flame resistant while maintaining clarity, by the addition to a novel polyvinyl chloride plastisol composition, of a phosphate plasticizer, according to the Sauer et al patent. The phosphate plasticizer is added in an amount of 3-50 parts by weight per 100 parts by weight of the polymer. While not part of the invention claimed by the Sauer et al patent, the reference does mention brominated monomeric triallyl phosphate as a flame retardant.
The novelty in the Levek et al patent resides primarily in the addition to a styrene based polymer, a stabilizer which reduces decomposition of the polymer by heat. The reference is relevant to the present invention because the polymer system also incorporates a flame retarding system made up of antimony trioxide and a bromine containing organic compound such as tetrabromobisphenol-A, octobromodiphenyl ether or the like. Levek et al also mentions that flame retardants such as tricresyl phosphate, zinc borate, ammonium polyphosphate, sodium antimonate and the like may be used in place of antimony trioxide.
The Rodish reference contributes to the art by virtue of a flame retardant for polystyrene made up of four materials viz a halogenated hydrocarbon, antimony trioxide, zinc borate, and a hydrate of alumina. The first three materials are well known flame retardants. The hydrated alumina, preferably the trihydrate, functions not as a flame or combustion suppressant but rather as a smoke depressant. Hydrated alumina, both the mono- and tri-hydrates when present in the polystyrene to the extent of 16-28 percent by weight, substantially depresses the amount of smoke generated when the polystyrene burns, by giving off its water of hydration. The hydrated alumina also restricts access of the polystyrene to oxygen thereby discouraging combustion. Obviously, this four component flame retardant does not eliminate polystyrene's combustibility but rather just reduces it as do the other multi component prior art flame retardant systems. The present invention does not totally eliminate the flammability characteristic of organic polymers but it does produce an unexpected further decrease in that property.